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as to why there isn't a standard, that's likely because a given weight is meaningless unless you know at least shutter speed, focal length, sensor size, and sensor resolution.
These are exactly the types of parameters that would be specified in any such standardized testing method.
I participate in various wireless protocol standards bodies (3GPP, 802.11, WFA). Both 3GPP and the Wi-Fi Alliance produce test plans for wireless products (cell phones, WiFi). In the case of WiFi, the vendors of this equipment are not required to submit their products for testing (unlike cell phones), but they do so voluntarily so that they can display the WiFi logo - a marketing plus for them as the WiFi logo has a lot of brand equity.
It would be nice if such a model existed for the tripod industry as well, but I suspect that industry is simply not large enough. Any such test/compliance organization would have to be funded by the vendors themselves (as members). And in order to make membership attractive, the logo a vendor would be allowed to display for complying with the test method would have to have some clout. But the logo will only have clout if membership reaches critical mass, so it is a catch 22.
Anyway, I still am curious to know in general what method the vendors currently use. Without such knowledge, my only rule of thumb now is to take the maximum weight of my equipment, double or triple it, and use that number as the minimum load capacity as stated by the manufacturer.